This invention relates to a medical instrument and to an associated method. More particularly, this invention relates to a medical instrument and method wherein power to an operative tip is automatically controlled.
Ultrasonic devices have been used to remove soft and hard tissue from mammalian bodies for over three decades, at least. These devices and methods for their use have been well documented in the art, such as U.S. Pat. No. 4,223,676 to Wuchinich, U.S. Pat. No. 4,827,911 to Broadwin and U.S. Pat. No. 5,419,761 to Narayanan et al. Applications include phacoemulsification, ablation of tumors in the liver and spine and subcutaneous removal of adipose tissue, also known as ultrasonic liposuction.
Most of the instruments used for these applications have several elements in common. These are an electrical generator which transforms line or battery power to relatively high voltage RF frequencies in the 20 kc to 100 kc range, a transducer of either a magnetostrictive or piezoelectric type and a probe or horn which is generally manufactured from titanium and amplifies the motion of the transducer from approximately 20 microns to over 400 microns in some cases. Means have been disclosed which allow the surgeon the ability to switch the output power on or off on demand. These include footswitch controls, finger or thumb switches on the handpiece or even by voice commands if the electronic generator includes the prerequisite software and electronics.
All of these means require that the surgeon coordinate the application or removal of ultrasonic power to the precise moment at which it is required. In spinal or brain surgery, this is not difficult, since the application of the power is not continuous and he or she has complete view of the operative site. By simply moving his hand, he is able to apply power to the surgical site and remove the tip of the probe even when the power is on, limiting the power input to tissue. In this way, tissue temperature rise is minimized and collateral damage is curtailed.
However, in applications such as liposuction, the surgeon would have a difficult time in controlling power requirements. In this case, the surgeon moves the handpiece with a piston like action, alternatively advancing and retracting the cannula in a predetermined pattern. See U.S. Pat. No. 5,527,273 for a more complete description of this action. Since the sides of the long cannula are in contact with the tissue at all times, power is being applied to the tissue as long as the probe is activated with ultrasonic energy. In actuality, the power is only required on the pushing stroke, since the ultrasonic power ablates the tissue in direct contact with the distal end of the probe. As the tissue disrupts, it liquefies and the cannula can be advance. In this way, channels or tunnels are created in the adipose tissue.
When the probe is pulled back to be repositioned and start another tunnel, the tissue contacts the side of the probe. Power is still being applied but tissue liquefaction does not occur, since it is the cavitation and shearing forces created by the probe tip that liquefies and emulsifies the cells. Therefore, this energy can be considered waste and it actually goes into tissue heating. The longer the probe is used, the higher the temperature rise will be. If the temperature rises above the necrosis point, burning, scarring and other deleterious effects will arise.
It would be difficult and tiring for the surgeon to time the on and off controls with his hand movements, since they are rapid and repetitive. It would be more desirable to have an automatic means to determine the power requirement and have the machine apply energy only when most needed.
Several embodiments for reducing the amount of ultrasonic power delivered to the tissue or samples have been known to the art for many years. One old technique is pulsing of the output. By automatically turning the output power on and off at specific duty cycles, the power may be reduced in inverse proportion to the output duty cycle. For instance, if the output power was turned on for 2/10ths of a second and shut off for the remainder of that second, it would be said the output power had a 20% duty cycle. At the beginning of the next second, the power is turned back on for 2/10ths of a second and so forth. The power input to the tissue or sample would be reduced by 80% over a given period. However, this embodiment does not turn off the power completely when not needed, i.e. on the return stroke, it only lessens it. In fact, the power is even reduced on the push stroke, when it is most needed increasing the effort needed to advance the cannula through the body.